patch 8.2.1982: quickfix window now updated when adding invalid entries

Problem:    Quickfix window now updated when adding invalid entries.
Solution:   Update the quickfix buffer properly. (Yegappan Lakshmanan, closes
            #7291, closes #7271)
diff --git a/src/quickfix.c b/src/quickfix.c
index ac95ace..40897b6 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -4674,14 +4674,17 @@
 	*dirname = NUL;
 
 	// Add one line for each error
-	if (old_last == NULL || old_last->qf_next == NULL)
+	if (old_last == NULL)
 	{
 	    qfp = qfl->qf_start;
 	    lnum = 0;
 	}
 	else
 	{
-	    qfp = old_last->qf_next;
+	    if (old_last->qf_next != NULL)
+		qfp = old_last->qf_next;
+	    else
+		qfp = old_last;
 	    lnum = buf->b_ml.ml_line_count;
 	}